Definition of chronic NO3-N toxicity thresholds for rainbow trout and other species cultured in WRAS is imperative because: (1) it provides a guideline for culture conditions that are conducive with optimal health, welfare, and performance of cultured species and (2) it establishes a critical water quality criterion that impacts the WRAS engineering design, including the makeup water flushing and feed loading rates, requirements for denitrification, as well as the waste water discharge volume and energy required to heat or cool the WRAS.